Detergent compositions in tablet form are described, for example, in GB 911204 (Unilever) and U.S Pat. No. 3,953,350 (Kao). They are sold commercially in Spain. Tablets have several advantages over powdered products: they do not require measuring and are thus easier to handle and dispense into the washload, and they are more compact, hence facilitating more economical storage. These detergent tablets are intended to be consumed completely when washing a single load. Thus they should disperse/dissolve completely when added to water.
Detergent tablets are generally made by compressing or compacting a detergent powder, which includes both an organic detergent active surfactant and detergency builder. It is desirable that tablets have adequate strength when dry, yet disperse and dissolve quickly when added to wash water.
Such tablets can be manufactured by stamping a chosen quantity of the detergent composition using a press with steel dies (also referred to as punches) which contact the powder and apply pressure so as to compact the powder into a tablet. Such a press may for example have two dies which move together within a surrounding sleeve, or one die which is driven towards a fixed anvil, again within a surrounding sleeve.
When making tablets with any kind of material, not necessarily detergent, a problem which can arise is adhesion of the composition to the steel mould parts. Adhesion of material to mould parts is disadvantageous, because the accumulated material spoils the surface finish of articles compacted in the mould. Such adhesion also interferes with the proper operation of production machinery which is constructed so as to monitor and control the compaction force employed. The traditional approaches to this problem of adhesion have been to provide a low adhesion and low friction surface on the mould parts, e.g. a conventional non-stick coating of polytetrafluoroethylene, or else to apply a release agent, for example magnesium stearate.
U.S Pat. No. 3,081,267 teaches that the dies should rotate relative to each other while compressing the composition, so as to prevent the composition from adhering to them. GB 2276345 teaches the stamping of articles, including tablets of compacted detergent powder, using mould parts surfaced with an elastomeric material of some thickness. The document explains that a suitable modulus of elasticity can be achieved with a surface coating of elastomer which is at least 0.5 mm thick. A range of 0.5 to 7 mm is disclosed. The thicknesses which are exemplified are about 4 mm but the surface coatings are shown as tapering to edges without appreciable thickness.
WO 97/20028 (Unilever) discloses the stamping of tablets using dies which carry a thin elastomer coating, which has a thickness not exceeding 0.5 mm over much or all of its area. This overcomes the problem of adhesion to the dies, and produces tablets with a smooth surface.
In co-pending US application serial number 09/059047, also published as WO 98/46720 on Oct. 22, 1998, it is disclosed that dies which carry a thicker elastomer coating lead to an increased speed of penetration of water into the tablets on immersion, thereby accelerating the dispersion/dissolution of the tablets.
The increased speed of water penetration is believed to arise because the elastomer layer on the dies leads to a tablet surface which is more permeable, and also somewhat less smooth, than the surface obtained from clean, plain steel dies, used to produce tablets of similar strength.